1. Field of the Invention
The present invention relates to a PIN photodiode formed from an amorphous semiconductor.
2. Description of the Prior Art
It is known to form PIN photodiodes from amorphous silicon.
Amorphous silicon is used, in preference to crystalline silicon, for different reasons. One of these reasons is that amorphous silicon has characteristics different from those of crystalline silicon and which, in certain uses, are more interesting.
Thus, the forbidden band width of crystalline silicon is from 1.1 e-V to 1.15 e-V, whereas that of amorphous silicon is from 1.6 to 1.75 e-V. Crystalline silicon provides detection in the visible range over a very great thickness with respect to that of amorphous silicon (a few tens of micrometers vs. 0.5 micrometer). Another reason for using amorphous silicon in preference to crystalline silicon is that it is possible to deposit large areas of amorphous silicon rapidly on a cheap material such as glass. On the contrary, crystalline silicon can only be deposited over small areas and that requires much higher temperatures.
Because of its disordered nature, in the crystallographic meaning of the term, amorphous silicon contains a large number of defects. These defects introduce so called localized states inside the forbidden band. Hydrogenated amorphous silicon is generally used which is a variant containing hydrogen which, by fixing itself on pendant links, that is to say unsatisfied, of silicon atoms reduces the density of the defects and improves the quality of the amorphous silicon.
Amorphous silicon is used more particularly in solar cells fitted for example to calculators and to detector strips, intended for telecopying.
The amorphous silicon PIN photodiodes which are known have numerous drawbacks. Among these drawbacks may be mentioned the fact that the P and N layers are obtained by doping, which considerably increases the number of defects of the amorphous silicon, since the number of defects is multiplied by a factor of the order of 100.
Another drawback is that, in order to obtain P or N type amorphous silicon, it is necessary to add to the silane, the thermal decomposition of which gives rise to amorphous silicon, a high concentration of the order of 5 to 10,000 ppm of a doping gas, such as diborane for obtaining the P type and phosphine for obtaining the N type. Now, diborane and phosphine are extremely dangerous gases in high concentration. The maximum dose tolerable for the human organism is 0.1 ppm for diborane and 0.3 ppm for phosphine, which involves very expensive safety measures for preventing leaks and treating the effluents from the depositing machines.
Another drawback of known amorphous silicon PIN photodiodes is that it is not possible to impinge the radiation to be detected indifferently on the N side or on the P side of the photodiode. In FIG. 1 an embodiment has been shown in which the radiation impinges on the P side. If it is desired to illuminate the N side, that is possible but requires modifying the structure of the photodiode so as to reduce the thickness of the N type layer so as to make it less absorbent. An additional drawback of heavy doping is that it makes the materials more absorbent.
Another drawback of the known amorphous silicon PIN photodiodes is that they have a high capacitance which may limit their use in photosensitive matrix structures, formed of interconnected photosensitive elements, in which each photosensitive element includes a PIN photodiode. These photosensitive structures may be used for detecting any type of rays, for example X rays when the photosensitive structure is preceded by a scintillator.
The present invention relates to PIN photodiodes formed from an amorphous silicon which, depending on the embodiments, overcome some or all the above mentioned drawbacks. They use the quantum effects of crystalline semiconductor multi-layers in the particular form they assume when the material is an amorphous semiconductor.
These multi-layers are described theoretically in the article by Inan Chen, published in "Physical review B", vol. 32, no. 2, July 15, 1985, page 885.
There exist two large categories of multi-layers. The first is that of the composition multi-layers in which there is superimposition of layers of different physico-chemical natures, doped or not. The second is that of doping multi-layers in which there is superimposition of layers of the same physico-chemical nature, but doped alternately P and N.